Since ink jet recording produces little or no noise, and makes high-speed recording possible using plain paper as the recording papers, ink jet recording has recently become widely used, e.g., for terminal output printers of computers. Also, multicolor recording can be easily performed by using plural ink nozzles, and multicolor ink jet recording by various ink jet recording systems have been investigated.
Examples of ink jet recording sheets used for ink jet recording include wood free papers, slip-writing continuous paper webs, art papers, coated papers, low density papers without size, ink jet recording papers having relatively good ink absorbing property and showing less blotting of ink as described in Japanese Patent Application (OPI) Nos. 53012/77, 74340/77 and 49113/78 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), fabric, plastic films having ink absorbing surfaces, wood boards, metallic plates, etc.
Ink jet recording (or printing) is generally applied to these ink jet recording sheets by aqueous inks. Aqueous inks for ink jet printing are typically composed of water-soluble dyes, humectants, dye-solubilizing agents, mold inhibitors, water, water-miscible organic solvents, etc., as described in Japanese Patent Application (OPI) Nos. 89534/74, 97620/74, 143602/75, 102407/75, 129310/76, 137506/76, 137505/76, 115106/76, 139408/76, 12008/77, 12009/77, 12010/77, 74406/77, 77706/78, 119107/78 and 119108/78, and Japanese Patent Publication Nos. 14643/77, 14644/77, and 20882/78.
Ink jet records obtained by applying conventionally known aqueous inks on the above-described known ink jet recording sheets exhibit the fault that when the records are splashed or wet with water, the records of dyes blot or diffuse completely due to the poor water resistance property thereof. Furthermore, when the records are preserved for a long period of time in a high humidity condition, the ink jet record also blots.
When an ink jet recording paper contains a dyeing component and the amount of jetted ink is small, as in the case of monochromatic ink jet recording, the water resistance properties of the records may be satisfactory for practical purpose if a dye or dyes having good water resistance properties are used. However, in the case of multicolor ink jet recording, the amount of jetted inks is relatively large, and records having sufficient water resistance properties cannot be obtained even when the ink jet recording paper contains good individual dye components. When papers recorded by ink jet printing are used, for example, for outdoor notifications or advertisements, the records are required to have particularly good water resistance properties but multicolor ink jet records formed by the combination of conventional ink jet recording papers and ink jet recording inks have been utterly unsuitable for such practical use.
Hitherto, dye mordants such as dicyandiamide condensates, polyamine, polyethyleneimine, etc., have been widely known as commercially available products, and when these mordants are incorporated in ink jet recording sheets, the water resistance properties of the images formed by ink jet recording increases considerably.
However, since the mordanting power of these mordants is limited, and the mordant itself has a high water-solubility, sufficiently high water resistance properties of ink jet records is not obtained even by using these mordants. In images formed by ink jet recording using aqueous ink jet recording inks, in particular, intermediate tone multicolor images formed by ink jet recording, if the dyes elute to even a slight extent, images will blot when the images are splashed or wet by water to drastically reduce the quality of the images. By using conventionally known mordants, it is difficult to completely prevent the dyes of images from dissolving into water, and thus it is difficult to render ink jet recording sheets water resistant.
Furthermore, in the case of multicolor ink jet recording, it frequently happens that inks are ejected from 2 or more nozzles of an ink jet printer and two or more ink dots overlap at various points on a recording paper, or in a more extreme case as many as 4 color ink dots may overlap at some points of the recording paper. Therefore, if in this case the prior ink drop is not quickly absorbed into the inside of the recording paper, the ink drop is mixed with the subsequent ink drop ejected to the same point and flows together from that point or is scattered by the subsequent ink drop to stain the white portions of the recording paper. Also, in such a case, if the records formed by jet ink recording are rubbed in handling of the recording paper, the ink drops remaining without being absorbed stain the recording paper. Therefore, recording papers having particularly excellent ink absorbing properties are required for successful multicolor ink jet recording.
However, a recording paper having good ink absorbing properties generally shows not only spreading of an ink dot in the planar direction of the recording paper, but also a large permeation of the ink dot into the inside of the recording paper. For example, a paper manufactured at a high bulk density without using size has a very high ink absorbing property, and such ink absorbing property is sufficiently enduring for multicolor ink jet recording, but has such faults as that (1) the ink dot on the paper spreads largely in a planar direction, reducing the resolving power of the image formed; (2) the ink permeating deeply into the inside of the paper reduces the density of the image and makes the image whitish, together with the scattering of light due to the voids in the paper. Thus, a clear, sharp image cannot be obtained. Also, when four-color image by cyan, magenta, yellow, and black inks is formed on a recording paper having good ink absorbing property by ink jet recording, the ink drops ejected first permeate most deeply into the inside of the paper to reduce the color reproducibility.
Thus, for obtaining color images having good color density, resolving power, and color reproducibility by multicolor ink jet recording, such properties, which are contrary to each other, that spreading of the coloring components in the ink drops to the longitudinal direction of a recording paper is less, permeation of the coloring components in the ink drops in the inside of the recording paper is less, the coloring components desirably exist on the surface of the recording paper, and the recording paper has good ink absorbing property, are required.
Various attempts have been made to overcome the foregoing problems. For example, Japanese Patent Application (OPI) No. 53012/77 discloses an ink jet recording paper wherein a coating material is applied on the surface of a base paper having a low degree of sizing and is permeated in the inside of the recording paper. Also, Japanese Patent Application (OPI) No. 49113/78 discloses an ink jet recording paper prepared by impregnating a paper containing a fine powder of a urea-formalin resin with a water-soluble polymer. Furthermore, Japanese Patent Application (OPI) No. 74340/77 discloses an ink jet recording paper having a specific gas permeability and having an appropriate ink absorbing time.
However, the techniques described in the foregoing patent applications are directed to improving the resolving power of images, the density thereof, etc., by sacrificing the ink absorbing property, and hence, although the resolving power, density, etc., of images formed may be improved to some extent, the ink absorbing property is reduced. Therefore, such papers are generally somewhat unsatisfactory as recording papers for multicolor ink jet recording. Thus, a continuing need has existed for recording sheets completely for multicolor ink jet printing.